Code allocation method in an MC-CDMA telecommunication system

ABSTRACT

A object of the present invention is to decrease or avoid the distortion of an MC-CDMA signal transmitted over a reverse link channel for a given amplifier efficiency. 
     The present invention concerns a code allocation method for a mobile telecommunication system. According to the invention, for each of a plurality of available spreading codes or available combinations of spreading codes a value of a first variable (PAPR k ) characteristic of the dynamic range of a modulated signal (S k ) is determined, and for each of a plurality of users a value of a second variable (α k ) characteristic of the propagation loss incurred over the transmission channel of the user is determined. A spreading code or combination of spreading codes producing a low dynamic range is allocated to said user if the propagation loss over its transmission channel is high.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for allocating a code to areverse link in a multi-carrier code division multiple access system.The present invention relates in particular to a method for allocating ascrambling/spreading code to such a reverse link.

2. Description of the Related Art

In recent years, Multi-Carrier Code Division Multiple Access (MC-CDMA)has been receiving widespread interest for wireless broadband multimediaapplications. Multi-Carrier Code Division Multiple Access (MC-CDMA)combines OFDM (Orthogonal Frequency Division Multiplex) modulation andthe CDMA multiple access technique. This multiple access technique wasproposed for the first time by N. Yee et al. in the article entitled“Multicarrier CDMA in indoor wireless radio networks” which appeared inProceedings of PIMRC'93, Vol. 1, pages 109-113, 1993. The developmentsof this technique were reviewed by S. Hara et al. in the articleentitled “Overview of Multicarrier CDMA” published in IEEE CommunicationMagazine, pages 126-133, December 1997.

Unlike DS-CDMA (Direct Spread Code Division Multiple Access), in whichthe signal of each user is multiplied in the time domain in order tospread its frequency spectrum, the signature here multiplies the signalin the frequency domain, each element of the signature multiplying thesignal of a different sub-carrier.

MC-CDMA combines the advantageous features of CDMA and OFDM, i.e. highspectral efficiency, multiple access capabilities, robustness inpresence of frequency selective channels, high flexibility, narrow-bandinterference rejection, simple one-tap equalisation, etc.

However MC-CDMA presents a significant drawback which is due to themulti-carrier modulation. Indeed, as shown below, an MC-CDMA signalconsists in a sum of modulated sub-carriers which may result in a highdynamic range.

More specifically, FIG. 1 illustrates the structure of an MC-CDMAtransmitter for a given user k. We consider here a reverse link, i.e. wesuppose that the transmitter is located in the mobile terminal of theuser. Let d^((k))(n) be the symbol to be transmitted from user k at timenT to the base station, where d^((k))(n) belongs to the modulationalphabet. The symbol d^((k))(n) is first multiplied at 110 by theproduct of a spreading sequence, denoted c^((k))(t), and a scramblingsequence specific to the user and denoted σ^((k))(t). The spreadingsequence consists of N “chips”, each “chip” being of duration T_(c), thetotal duration of the spreading sequence corresponding to a symbolperiod T. Without loss of generality, we assume otherwise specified inthe following that a single spreading sequence is allocated to the user.In general, a user may be allocated one or a plurality of orthogonalspreading sequences, according to the data rate required. In order tomitigate cellular interference (inter-cell interference and intra-cellinterference), the spreading/scrambling sequences allocated to differentusers are preferably chosen orthogonal.

The results of the multiplication of the symbol d^((k))(n) by theelements of the product sequence are multiplexed over a subset offrequencies of an OFDM multiplex. In general the number N of frequenciesof said subset is a sub-multiple of the number L of frequencies of theOFDM multiplex. We denote Ω_(k) the subset of {0, . . . , L−1} indexingthe frequencies used by user k, c_(l) ^((k)), l∈Ω_(k) the values of thecorresponding spreading sequence elements σ_(l) ^((k)), l∈Ω_(k) thevalues of the scrambling sequence elements. The block of symbolsmultiplexed in 120 is then subjected to an inverse fast Fouriertransformation (IFFT) in the module 130. In order to prevent intersymbolinterference, a guard interval of length typically greater than theduration of the impulse response of the transmission channel, is addedto the MC-CDMA symbol. This is achieved in practice by adding a prefix(denoted Δ) identical to the end of the said symbol. After beingserialised in the parallel to serial converter 140, the MC-CDMA symbolsare amplified in amplifier 150 in order to be transmitted over thereverse link transmission channel. The MC-CDMA method can therefore beanalysed into a spreading in the spectral domain (before IFFT) followedby an OFDM modulation.

The signal S_(k)(t) at time t which is supplied to the amplifier beforebeing transmitted over the reverse link transmission channel cantherefore be written, if we omit the prefix:

$\begin{matrix}{{S_{k}(t)} = {{{d^{(k)}(n)}{\sum\limits_{l \in \Omega_{l}}{c_{l}^{(k)}\sigma_{l}^{(k)}{\exp\left( {j\; 2\pi\; f_{l}t} \right)}\mspace{14mu}{for}\mspace{14mu} n\; T}}} \leq t \leq {\left( {n + 1} \right)T}}} & (1)\end{matrix}$where f_(l), l=0, . . . ,L−1 are the frequencies of the OFDM multiplex.

The dynamic range of the MC-CDMA signal S_(k)(t) is estimated by theso-called Peak to Average Power Ratio (PAPR) expressed by:

$\begin{matrix}{{{PAPR}\left( S_{k} \right)} = \frac{\max\limits_{T_{m}}\left| {S_{k}(t)} \right|^{2}}{\left. {\frac{1}{T_{m}}{\int_{0}}^{T_{m}}} \middle| {S_{k}(t)} \middle| {}_{2}{\mathbb{d}t} \right.}} & (2)\end{matrix}$where T_(m) is the time window over which the MC-CDMA signal isobserved. Equivalently, the dynamic range of the MC-CDMA signal isexpressed by the so-called Crest Factor (CF), simply defined as:CF(S _(k))=√{square root over (PAPR(S _(k)))}  (3)An MC-CDMA signal of large PAPR is particularly sensitive tonon-linearities of the output amplifier, also referred to as High PowerAmplifier (HPA). Indeed, above a given signal amplitude, the HPA entersa saturation zone and the amplified signal is significantly distorted.The level of distortion of the amplified signal A_(k)(t) is expressed bythe so-called Output Back-Off (OBO):

$\begin{matrix}{{OBO}_{k} = \frac{P_{sat}}{E\left( \left| {A_{k}(t)} \right|^{2} \right)}} & (4)\end{matrix}$where E(|A_(k)(t)|²) is the mean power of the amplified signal andP_(sat)=A_(sat) ² represents the saturation power of the amplifier whereA_(sat) is the amplitude saturation threshold. Equivalently, the levelof distortion can be assessed at the input of the amplifier by theso-called Input Back-Off (IBO):

$\begin{matrix}{{IBO}_{k} = \frac{E\left( \left| {S_{k}(t)} \right|^{2} \right)}{P_{sat}}} & \left( 4^{\prime} \right)\end{matrix}$where E(|S_(k)(t)|²) is the mean power of the input signal.

The characteristics of an HPA amplifier is shown in FIG. 2 where I(t)and O(t) respectively denotes a signal amplitude at the input and theoutput of the amplifier. An example of MC-CDMA signal S_(k)(t) is alsorepresented at the input of the amplifier. It should be noted that, bydecreasing the output back-off, the operating point of the amplifier isshifted towards saturation and for a given OBO threshold non-lineardistortion appears. The lower the output back-off, the higher thedistortion but also the higher the amplifier efficiency. The outputback-off has therefore to be adjusted to an optimal value in order toobtain the best efficiency for a given level of distortion.

SUMMARY OF THE INVENTION

A first object of the present invention is to decrease or avoid thedistortion of an MC-CDMA signal transmitted over a reverse link channelfor a given amplifier efficiency. A second object of the presentinvention is to increase the amplifier efficiency for a given distortionlevel of said MC-CDMA signal.

The basic idea underlying the invention is to allocate a code to a userby taking into account, on the one hand, the signal attenuation over theuplink channel and, on the other hand, the different PAPRs relative tothe available codes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts schematically the structure of an MC-CDMA transmitterknown from the state of the art;

FIG. 2 depicts schematically the amplitude characteristics of a HighPower Amplifier;

FIG. 3 depicts schematically a flow chart of the code allocationprocedure according to an embodiment of the invention;

FIG. 4 depicts schematically a method for setting the amplifier of amobile terminal when a code allocation procedure according to anembodiment of the invention is used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

We refer back again to the context of an MC-CDMA telecommunicationsystem and we assume that each user k=1, . . . ,K can be allocated oneor a plurality I_(k) of spreading codes c^((k,i)), i=1, . . . ,I_(k)where c^((k,i)) is the sequence represented by c_(l) ^((k,i)), l∈Ω_(k,i)and Ω_(k,i) the subset of carriers of the OFDM multiplex which is usedby the spreading code c^((k,i)). In general, the subsets Ω_(k,i)relative to a given user are chosen identical, that is Ω_(k,i)=Ω_(k). Insuch instance, the spreading sequences c_(l) ^((k,i)), i=1, . . . ,I_(k)are chosen orthogonal. The subsets may also be chosen disjoint,typically as interleaved subsets of {0, . . . ,L−1}. For instance, iftwo spreading c^((k,1)) and c^((k,2)) are used for user k, the firstsubset Ω_(k,1) may correspond to the odd values of l and the secondsubset Ω_(k,2) to the even values. In the latter case, the spreadingsequences need not be orthogonal since they are frequency multiplexed.

Given I_(k) consecutive symbols d^((k))(n),d^((k))(n+1), . . .,d^((k))(n+I_(k)−1) having to be transmitted by user k, the MC-CDMAsignal carrying these symbols can be expressed, similar to (1):

$\begin{matrix}{{S_{k}(t)} = {{\sum\limits_{i = 1}^{I_{k}}{{d^{(k)}\left( {n + i} \right)}{\sum\limits_{l \in \Omega_{k,i}}{c_{l}^{({k,i})}\sigma_{l}^{(k)}{\exp\left( {j\; 2\pi\; f_{l}t} \right)}\mspace{14mu}{for}\mspace{20mu}{nT}}}}} \leq t \leq {\left( {n + 1} \right)T}}} & (5)\end{matrix}$

As already seen above, the dynamic range of S_(k)(t) can be estimated byits PAPR. The value PAPR(S_(k)) depends among others upon the spreadingcodes c^((k,i)) and the scrambling code σ^((k)) allocated to the user.

In an uplink MC-CDMA telecommunication system, a base station or aplurality of neighbouring base stations are allocated a set of spreadingcodes. The base station allocates to each user k within the cell, one ora plurality of spreading codes (according to the data rate required) andone scrambling code.

The procedure for allocating a scrambling code and spreading code(s) toa given user k is illustrated in FIG. 3.

Let us consider a base station to which a set of spreading codes {Λ₁, .. . ,Λ_(N)} and a set of scrambling codes {Γ₁, . . . ,Γ_(P)} areallocated (step 310), for example by a RNC (Radio Network Controller).

In step 320, the base station estimates for each combination ofspreading codes and scrambling code (Λ_(n) ₁ ,Λ_(n) ₂ , . . . ,Λ_(n)_(s) , Γ_(p)) where Λ_(n) _(i) ∈{Λ₁, . . . ,Λ_(N)} and Γ_(p)∈{Γ₁, . . .,Γ_(P)} the maximum value of PAPR(S_(k)), that is:

$\begin{matrix}{{PAPR}_{k} = {\underset{S_{k}}{Max}\left\lbrack {{PAPR}\left( S_{k} \right)} \right\rbrack}} & (6)\end{matrix}$where the maximum is taken over the sequences of consecutive symbolsthat can be transmitted by the user.

The combinations (Λ_(n) ₁ ,Λ_(n) ₂ , . . . ,Λ_(n) _(s) ,Λ_(p)) are thensorted in 330 according to their corresponding PAPR_(k) values andstored in a look-up table Ξ.

Alternatively, according to a preferred embodiment of the invention, thePAPR_(k) values having been predetermined, the combinations (Λ_(n) ₁,Λ_(n) ₂ , . . . ,Λ_(n) _(s) ,Γ_(p)) are pre-stored in a look-up tableaccording to the ascending or descending PAPR_(k) order. In suchinstance, the steps 320 and 330 are simply omitted.

It is assumed that the base station can estimate (340) the pathattenuation for each and every active user k, for example, from a powercontrol information transmitted by the mobile terminal. Alternatively,the path attenuation can be derived from the distance of the mobileterminal to the base station. For example, the path attenuation can beexpressed in terms of attenuation coefficient α_(k) or distance valued_(k).

The base station sorts (step 350) the propagation path attenuationcoefficients α₁, . . . ,α_(K) or the distances relative to the K activeusers of the cell. Without loss of generality we may suppose that α₁≦ .. . ≦α_(k)≦ . . . ≦α_(K). We suppose also that the same number ofspreading codes is allocated to the K users, i.e. I_(k)=I, ∀k. If thisis not the case, the users are sorted and clustered into groups of usersrequiring the same number of spreading codes and the allocationprocedure is carried out for each group independently.

According to a first embodiment of the invention, the allocationprocedure allocates to each user k a (I+1)-tuple (Λ_(n) ₁ ,Λ_(n) ₂ , . .. ,Λ_(nI) _(k) ,Γ_(p)) consisting of I spreading codes and onescrambling code, such that the ordering of the PAPR_(k) values is asfollows:PAPR_(K)≦ . . . ≦PAPR_(k)≦ . . . ≦PAPR₁  (7)

In other words, the codes are allocated so that the more distant users(or the users suffering from a higher propagation loss) benefit from thecodes generating lower PAPR values. By so doing, the HPA of a distantmobile terminal can be operated at a lower output back-off value, whichin turn allows to reduce the distortion level for a given amplifierefficiency or, conversely, to increase the amplifier efficiency (andhence to lower the power consumption) for a given distortion level. Afurther advantage of the invention lies also in a larger cell coverage(i.e. a larger cell diameter) since the distance of a mobile terminal tothe base station can be increased for a same power consumption and agiven distortion level.

The code allocation procedure set out above is carried out at regulartime intervals for tracking the changes in the propagation pathattenuation coefficients of the different users. However, in order toavoid frequent re-allocation of the codes, it can be decided that nore-allocation is effected if the variations of the propagation pathattenuation coefficients lie below a given threshold. The codeallocation procedure is also carried out each time a link to beestablished or released, e.g. during handover.

Preferably, the look-up table Ξ mentioned above is scanned, startingfrom the (I+1)-tuple of lowest PAPR value and the codes stored thereinare allocated to the users starting from the one suffering from thehighest propagation loss.

For a given user k, the spreading codes Λ_(n) ₁ ,Λ_(n) ₂ , . . . ,Λ_(nI)_(k) and the scrambling code Γ_(p) retrieved from the table areallocated to the user (step 360) as follows:c ^((k,i))=Λ_(n) ₁ and σ^((k))=Γ_(p)  (8)

According to a second embodiment of the invention, the range ]0,α_(max)]of the attenuation coefficients where α_(max) (or similarly the distancerange ]0,R_(max)] where R_(max) is the cell radius) is split up into Melementary ranges ]0,ρ₁],]ρ₁,ρ₂], . . . ,]ρ_(M−1),α_(max)]. Eachelementary range ]ρ_(m−1),ρ_(m)] is attributed a subset Ξ_(m) of(I+1)-tuples (Λ_(n) ₁ ,Λ_(n) ₂ , . . . ,Λ_(nI) _(k) ,Γ_(p)) such thatthe PAPR value resulting from any combination of codes belonging to asubset Ξ_(m) is lower than the PAPR value generating from anycombination of codes belonging to the subset Ξ_(m−1).

For a given user k, the allocation procedure first determines in whichelementary range ]ρ_(m−1),ρ_(m)] the attenuation coefficient falls. Anavailable combination of codes is then looked for in the subset Ξ_(m).Advantageously, the subset Ξ_(m) is arranged as a look-up table storedin a memory of the base station. Preferably, the look-up table Ξ_(m) isscanned, starting from the (I+1)-tuple of lowest PAPR value and thefirst combination of available of codes is allocated to the user. The(I+1)-tuple is then marked as unavailable in the table until a newallocation makes it available back again.

Here again, the code allocation procedure is carried out at regular timeintervals and each time a user requests to establish or release a link.However, in the present embodiment, provided the attenuation coefficientof a user remains in the same elementary range no re-allocation isneeded.

According to a first variant, an information indicating the allocatedspreading code(s) and scrambling code is sent to the user (step 370).Preferably, the look-up table Ξ (or the set of look-up tables Ξ_(m)) isalso stored in a memory of the mobile terminal and the informationindicating the allocated codes is an address of said table.

According to a second variant, in addition to the information indicatingthe allocated codes, the base station transmits to the mobile terminalan information giving the PAPR_(k) value corresponding to said allocatedcodes. It should be noted that in place of PAPR_(k) the Crest Factor√{square root over (PAPR_(k))} or, more generally, an informationcharacteristic of the dynamic range of the modulated signal can betransmitted.

According to a third variant, in addition to the information indicatingthe allocated codes, the base station transmits to the terminal aninformation giving the optimal output back-off, denoted OBO_(k),corresponding to said PAPR_(k) value. In such instance, however, thebase station needs to know the characteristics of the HPA of the mobileterminal.

As shown in FIG. 4, on the mobile terminal side, the informationindicating the allocated codes is received in 410. The allocatedspreading codes c^((i,k)) and scrambling code σ^((k)) are retrieved fromsaid information and the corresponding value PAPR_(k) is calculated in420. The output back-off value OBO_(k) is derived therefrom in 430 andthe operating point of the amplifier is set accordingly (step 440).

In the second variant of the invention, the calculation step 420 isskipped (since the PAPR_(k) is sent by the base station to the mobileterminal) and it is directly proceeded with the calculation of OBO_(k)and the setting of the operating point of the amplifier.

Similarly, in the third variant of the invention, the calculation steps420 and 430 are skipped and it is directly proceeded with the setting ofthe operating point of the amplifier. Furthermore, it should be notedthat the optimal input back-off value IBO_(k) of the amplifier and moregenerally an information representative of the optimal setting of theoperating point of the amplifier can be transmitted in place of theoptimal output back-off value OBO_(k).

Although the invention has been essentially described in the foregoingas a code allocation method for an MC-CDMA telecommunication system, itshould be clear to the man skilled in the art that it can also beapplied to any system combining code multiple access and OFDMmodulation.

1. Code allocation method for a mobile telecommunication system in whichthe symbols sent from a user (k) to a base station are spread with aspreading code or a combination of spreading codes (c^((k,i))) beforebeing modulated with a plurality (Ω_(k,i)) of frequency carriers toproduce a modulated signal (S_(k)) transmitted to said base station overa transmission channel, characterised in that for each of a plurality ofavailable spreading codes or available combinations thereof (Λ_(n) ₁ ,Λ_(n) ₂ , . . . ,Λ_(nI) _(k) ) a value of a first variable (PAPR_(k))characteristic of the dynamic range of said modulated signal (S_(k)) isdetermined, and that for each of a plurality of users a value of asecond variable (α_(k)) characteristic of the propagation loss incurredover the transmission channel of the user is determined, and that aspreading code or combination of spreading codes producing a highdynamic range is allocated to a user if the propagation loss over itstransmission channel is low, whereas a spreading code or combination ofspreading codes producing a low dynamic range is allocated to said userif the propagation loss over its transmission channel is high.
 2. Codeallocation method according to claim 1, characterised in that thesymbols sent by a user to said base station being multiplied with ascrambling code before being modulated, characterised in that for eachof a plurality of available spreading codes or available combinationsthereof (Λ_(n) ₁ ,Λ_(n) ₂ , . . . ,Λ_(nI) _(k) ) and each of a pluralityof scrambling codes (Γ_(p)) a value of a first variable (PAPR_(k))characteristic of the dynamic range of said modulated signal (S_(k)) isdetermined, and that for each of a plurality of users a value of asecond variable (α_(k)) characteristic of the propagation loss incurredover the transmission channel of the user is determined, and that ascrambling code and a spreading code or combination of spreading codesproducing a high dynamic range is allocated to a user if the propagationloss over its transmission channel is low, whereas a scrambling code anda spreading code or combination of spreading codes producing a lowdynamic range is allocated to said user if the propagation loss over itstransmission channel is high.
 3. Code allocation method according toclaim 2, characterised in that the values of said first variable for thedifferent spreading codes or combinations of spreading codes and thedifferent scrambling codes, if used, are sorted according to a firstorder.
 4. Code allocation method according to claim 3, characterised inthat the users served by said base station are sorted according to asecond order, said second order being the same as the order of therespective values of said second variable for said users.
 5. Codeallocation method according to claim 4, characterised in that, spreadingcodes/scrambling codes are allocated to said users sorted according tosaid second order, so that the values of the first variable for saidallocated codes are arranged in said first order.
 6. Code allocationmethod according to claim 1, characterised in that the variation rangeof the second variable is split up into consecutive elementary rangesand that the users are sorted according to a third order, said thirdorder being the same as the order of the elementary ranges in which saidvalues of the second variable respectively fall.
 7. Code allocationmethod according to claim 1, characterised in that said first variableis the maximum of a peak to average ratio of the modulated signal. 8.Code allocation method according to claim 1, characterised in that saidsecond variable is an attenuation coefficient of the transmissionchannel.
 9. Code allocation method according to claim 1, characterisedin that said second variable is a location information of the user. 10.Code allocation according to claim 1, characterised in that said basestation transmits to said user a first information indicating the codeor the codes allocated thereto.
 11. Code allocation according to claim10, characterised in that said base station transmits to said user asecond information (PAPR_(k)) giving the value of the first variable forthe code or codes allocated to said user.
 12. Code allocation accordingto claim 10, characterised in that said base station derives, from thevalue of the first variable for the code or codes allocated to saiduser, a third information representative of a setting of an amplifierfor amplifying the modulated signal of said user and transmits saidthird information to said user.